1. Field of Invention
The present invention relates to nuclear magnetic resonance techniques for measuring parameters of a sample, particularly in inhomogeneous magnetic fields.
2. Discussion of Related Art
In standard nuclear magnetic resonance (NMR) spectroscopy, the primary information of interest is contained in the spectrum of the signal. This is made possible because magnets are now available with homogeneities typically better than 1 part in 108. However, some applications require large samples which are unable to fit inside standard superconducting magnets, and thus require the use of one-sided magnet systems. As a result, the magnetic field across these samples is necessarily inhomogeneous and the signal-to-noise ratio is intrinsically low. One such application is in the field of oil-well logging.
A natural scale by which to measure inhomogeneities in the static field, B0, is the amplitude of the RF field B1. In this disclosure, the term grossly inhomogeneous fields is used to describe those fields in which the inhomogeneities of the static field, ΔB0, exceed the strength of the RF field, B1. In this case, the NMR signal spectrum depends mainly on B1 and the value of the dephasing time of the free induction decay, T*2, is on the order of the pulse duration. This implies that standard NMR spectroscopy cannot be used encode chemical or spatial information in the signal spectrum. As a consequence, the standard “spectral approach” fails with downhole NMR logging devices that have grossly inhomogeneous fields.
Spin relaxation times, such as the longitudinal relaxation time, T1, and the transverse relaxation time, T2, are important for characterization of crude oils. Most NMR logging measurements are currently based on measurements of transverse relaxation times, T2, because they can be measured very efficiently using a Carr-Purcell-Meiboom-Gill (CPMG) pulse sequence. The CPMG sequence generates a long train of echoes whose amplitudes decay with the time constant T2. In this case, the echo amplitudes provide the essential information, in particular, the initial amplitude and the decay time. The measurement of the longitudinal relaxation time, T1, is more time consuming. However, there are circumstances when T1 measurements may be more desirable than T2 measurements, particularly when the intrinsic relaxation times are long, for example, greater than one second. In such cases, the intrinsic T2 sometimes cannot be determined because the measurement becomes dominated by diffusion and motion effects, whereas in contrast, T1 is not affected by diffusion and is less affected by motion effects.
A large number of T1 measurement techniques can be found in the literature. The majority of schemes are based on inversion recovery or saturation recovery. This requires measurements with long recovery times (e.g., greater than several times T1) to determine the equilibrium magnetization, M0. In samples with long T1 this results in very lengthy measurement cycles. There are many techniques that attempt to speed up measurements of T1, such as measuring the approach to steady-state magnetization, progressive saturation measurements (in which a series of steady state signals with different relaxation weightings are measured), speed-optimized fast inversion recovery (FIR) methods, and many others. However, common to most existing methods is the requirement to take one or several data for recovery times much longer than T1 in order to obtain the equilibrium signal.
Single-scan measurements are the fastest T1 measurement schemes. Many of these schemes are a modification of the so-called “triplet method,” in which the recovering longitudinal magnetization is monitored by briefly converting it into transverse magnetization, detecting it, and the restoring it back to longitudinal magnetization. However, in grossly inhomogeneous fields, off-resonance effects prevent complete conversion into transverse magnetization and back. As a result, the measured relaxation time is not a pure T1 relaxation time, but with a strong admixture of T2. Another single-scan approach to measure T1 is based on a standard two-dimensional inversion-recovery sequence, but the second dimension is encoded in the spatial dimension using pulsed field gradients. This allows the second dimension to be encoded simultaneously with the first dimension, to reduce the measurement time to that of a one-dimensional experiment. However, this technique is also not easily adapted to grossly inhomogeneous fields.